Heifer reproduction

Definition

Heifer reproduction covers the breeding performance of nulliparous heifers (never calved, LACT=0 in DC305) from first insemination to confirmed pregnancy. The measures are the same ideas as for cows, with age in place of days in milk:

Heifer conception rate: pregnant services ÷ services with a known outcome, as in conception rate.
Heifer 21-day pregnancy rate: pregnancies ÷ eligible heifers per 21-day period, as in pregnancy rate. Eligibility starts at the heifer VWP (below), not at calving.
Heifer insemination (heat detection) rate: share of eligible heifers bred per 21-day period.
Age at first service (AFS): age in days or months at first insemination.
% pregnant by target age: share of a birth cohort pregnant by a set age, for example 15 months (about 455 days). This sets up age at first calving.

Heifers usually have much higher conception rates than lactating cows, and they are where many herds use sexed semen, so heifer and cow numbers must always be reported separately.

Heifer VWP by age

Heifers have no calving date, so their “voluntary waiting period” is an age (and often a body-weight) threshold: the age at which heifers become eligible for breeding. Penn State Extension’s target is 55% of mature body weight (often 750–800 lb for Holsteins) and 90% of mature structural growth by a breeding age of 13–15 months; the 55% target follows NRC 2001. Herds with fast-growing heifers often start at 12–13 months once heifers reach that weight.

The herd’s written policy is often not what happens in the barn. A practical data-driven definition: the age at which the first 10% of a cohort has received a first service. That age is a good estimate of when breeding actually starts, and it gives a start date for the heifer pregnancy-rate eligibility window. State which definition was used.

Formula

Plain text:

Heifer CR (%) = pregnant heifer services / (pregnant + open heifer services) × 100

Heifer PR (%) = heifers pregnant in the 21-day period / heifers eligible in the period × 100
                (eligible = past heifer VWP age, not pregnant, not DNB, in the herd)

Age at first service (days) = first service date − birth date

% pregnant by target age = heifers in the birth cohort pregnant by that age / heifers in the cohort still in the herd at that age × 100

LaTeX:

\mathrm{CR}_{\text{heifer}} = \frac{N_{\text{preg}}}{N_{\text{preg}} + N_{\text{open}}} \times 100
\qquad
\mathrm{AFS}_i = d_{\text{first service},i} - d_{\text{birth},i}
\qquad
\%\mathrm{Preg}_{\le A} = \frac{N_{\text{preg by age }A}}{N_{\text{in herd at age }A}} \times 100

For % pregnant by target age, decide how to treat heifers sold or dead before the target age. Leaving them out of the denominator (as above) measures breeding performance; including them measures replacement supply. Say which.

How to pull it in DC305

Heifers are LACT=0. Heifer breeding is only in DC305 if the herd records heifer breedings there; some herds keep heifers in a separate file or at a grower.

BREDSUM\Y                               heifer CR, all services
BREDSUM\YB                              heifer CR by service number (times bred)
BREDSUM\YE                              heifer 21-day table (HDR and PR)
BREDSUM\YS                              heifer CR by service sire
BREDSUM\Y*                              heifer CR by semen type

BREDSUM reports on heifers through the \Y (youngstock) switch, combined with the table switch, rather than through FOR LACT=0. In the 21-day table, a Select Sires guide (2013) sets the heifer VWP as an age in days, e.g. BREDSUM\YERV390 (eligible from 390 days of age).

Age at first service and pregnancy by age (AGE is age in months, BDAT birth date, RC 5 pregnant):

LIST ID BDAT AGE TBRD RC FOR LACT=0 AGE>13            heifers over 13 months, breeding status
COUNT ID FOR LACT=0 AGE>15 RC=5                        pregnant heifers over 15 months
COUNT ID FOR LACT=0 AGE>15                             all heifers over 15 months
SUM AGEFB FOR LACT=0 TBRD>0                            average age at first breeding

AGEFB (age at first breeding) appears in the VAS and Select Sires analysis examples but is not in every setup; check the herd’s item list.

Sexed vs conventional semen is identified by the semen type on the service (BREDSUM\Y*), or by sire code or a breeding remark. How it is entered varies by herd: look it up in the herd’s dc305-vocab mapping.

Typical range

Herd context: US Holstein heifers, group-housed (freestall, dry-lot or bedded pack), bred by AI with a mix of detected heats and timed AI.

Measure	Typical	Good
Heifer CR, conventional semen	55–65%	> 65%
Heifer CR, sexed semen	45–55%	> 55%
Heifer 21-day PR	30–45%	> 45%
Age at first service	13–14 months	12.5–13.5 months
% pregnant by 15 months (455 days)	70–85%	> 90%
Age at first calving	23–25 months	22–23 months

Published reference points: Holstein heifers bred to Holstein sires averaged 55.3% CR in US national records (McWhorter et al. 2020); first-service CR with sexed semen averaged 47% in Holstein heifers (DeJarnette et al. 2009).

Sexed semen usually gives a heifer CR about 75–90% of conventional (for example 50% vs 60%). In US on-farm records it was about 80% of conventional in the same herds (DeJarnette et al. 2009).
Heat stress hurts heifers less than lactating cows, but summer CR still drops in hot regions.

Common mistakes

Mixing heifers with cows in CR or PR. Heifer CR is 15–25 points higher; a blended number moves with the mix.
Ignoring sexed semen. Compare heifer CR within semen type, not pooled.
No heifer VWP in DC305 (or the wrong one). Without an age-based eligibility start, the heifer 21-day table is meaningless.
Using written policy as the VWP. Use the age at which breeding actually starts (for example the 10th-percentile age at first service).
Off-site heifers. If heifers are bred at a grower and their breedings are not entered, DC305 shows them as unbred.
Tracking age only. Breeding small heifers at the target age gives poor results; pair age targets with body weight or height.
Treating % pregnant by 15 months as a cow-level number. It is a cohort measure; state the birth cohort and how culled heifers were handled.